A control valve regulates the rate of fluid flow as the position of a valve plug or disk is changed by force from an actuator. To do this, the control valve must: (1) contain the fluid without external leakage; (2) have adequate capacity for the intended service; (3) be capable of withstanding the erosive, corrosive, and temperature influences of the process; and (4) incorporate appropriate end connections to mate with adjacent pipelines and actuator attachment means to permit transmission of actuator thrust to the valve plug stem or rotary shaft, for example.
Many styles of control valves are known. For example, sliding-stem control valves and rotary-shaft control valves are well known in the field of process control valves. Sliding-stem control valves include globe valves, which are valves having a linear motion closure member, one or more ports, and a body distinguished by a globular shaped cavity around a port region. Sliding-stem control valves typically employ a plug for their closure member that is positioned in the flow path to modify the rate of flow through the valve. The sliding-stem control valve further includes a valve stem having a first end connected to the closure member and a second end opposite the first end that is connected to an actuator.
A rotary-shaft control valve is a valve in which the flow control member, such as a full ball, partial ball, sphere or disk, is rotated in the flowstream to control the capacity of the valve. The rotary-shaft control valve includes a shaft that corresponds to the valve stem of a globe or sliding-stem valve.
Some methods of detecting fatigue and an onset of cracking in a shaft of a rotary-shaft control valve or a stem of a sliding-stem control valve are known. For example, it is known to mount strain gauges on control valve stems and shafts to verify that a force or torque has been transmitted to the valve once a positioner and an actuator have been given a control signal to close.
It is desired, however, to detect cracks in the valve stem or shaft that are too small to be identified by visual inspection, for example. It is also desired to detect cracks and fatigue in the valve stem and shaft earlier and more accurately than current methods allow. By doing so, end users are alerted that the control valve is in need of replacement parts and service, promoting more efficient maintenance and a longer useful life of the control valve.